Heretofore, ultrahigh molecular weight polyethylene has been known to be superior to general-purpose polyethylene in impact resistance, self-lubricating properties, abrasion resistance, sliding properties, weather resistance, chemical resistance, dimensional stability, etc. and to have physical properties comparable to engineering plastics. However, because of its high molecular weight, ultrahigh molecular weight polyethylene is poor in fluidity when melted, and is hardly moldable by kneading extrusion unlike usual polyethylene having a molecular weight within a range of from a few tens thousands to about 500,000. Therefore, ultrahigh molecular weight polyethylene is molded by a method of directly sintering a polymer powder obtained by polymerization, a method of compression molding, an extrusion molding method by means of a ram extruder to carry out extrusion molding while exerting compression intermittently, or a method of extrusion molding in a state dispersed in e.g. a solvent, followed by removing the solvent.
However, such molding methods have had a problem such that the technical level of difficulty is so high that it is difficult to obtain molded products, and further, due to the presence of locally highly viscous portions caused by entanglement of high molecular chains or due to deficiency in flowability of polymer particles, non-dense portions are likely to be formed to result in weak points, whereby it tends to be difficult to attain mechanical strength which the obtainable molded product is expected to have, and the mechanical strength tends to be relatively low.
On the other hand, as a means to increase the mechanical strength at the time of being formed into a molded product, ultrahigh molecular weight polyethylene having a narrow molecular weight distribution, using a metallocene catalyst, has been proposed (see e.g. Patent Documents 1 and 2).